Method of fractionating hydrocarbon oils



June 15, 1943. w. w. KRAFT ETAL 2,321,826

METHOD OF FRACTIONATING HYDROCARBON OILS Filed May 13, 1941' INVENTORS#7261150! MZ/fr ,J

BY g1 Patented June 15, 1943 METHOD OF 'FRACTIONATING HYDEQ- CARBON OILSWheaten W. Kraft, Scarsdale, N. -Y., and -Walter L. Bass, East Orange,N. J., assignors to .The Lummus Company, New York, N. Y., a corporationof Delaware Application May 13, 1941, Serial No. 393,136

9 Claims.

The present invention relates to the fractional distillation ofhydrocarbon oils such as petroleum and the like and more particularly toa method of improving the quality of the several fractions obtainedtherefrom.

In the operation of a fractionating column for hydrocarbons, it iscommon practice to withdraw one or more side streams from thecolumn atdifierent levels and to introduce such side streams into stripping unitsindividual to each stream. As is well known, such side streams,represent particular fractions of the hydrocarbon mixture beingdistilled. The operation of the stripping units is in the nature of afurther distillation of the side streams. The purpose of suchsupplemental distillation is to free the respective side streams ofretained low boiling constitutents commonly called light ends.

These light ends deleteriously affect the distillation characteristicsand more especially the flash point of the oil Withdrawn as a finishedproduct. Consequently, the further the stripping operation is carriedout on a given fraction comprising one of the side streams, the greaterwill be the improvement in the distillation specifications of theresulting product. Naturally, there are practical limitations beyondwhich it becomes uneconomical to extend the stripping operation.

The operation of these stripping units is effected by vaporization ofthe undesirable light ends from the side stream, which vaporization maybe produced according to various methods commonly employed in the art.One such method includes the introduction of low-boiling vapors from themain fractionating column into the stripping column countercurrently tothe withdrawn side stream with or without the supply of heat at the baseof the stripping column. This method has the objection that thecomposition of the recycled vapors is too close to that of the materialwhich it is desired to strip out in order to permit effective stripping.

Another method has been the production of a partial pressure effect bythe introduction of steam into the stripping column. According to thismethod, it is necessary to maintain a rather close relation between thecooling effect resulting from the vaporization of the low boilingcomponents and the partial pressure effect of the steam in order tosecure efficient operation in the stripping column. That is to say,where steam stripping is used, vaporization of the light endspresupposes cooling; and, if the cooling effect is permitted tooveroalance the partial pressure effect of the steam, an eilectivestripping atmosphere no longer prevails in the column. This conditiontends to become aggravated so that it is necessary to use a relativelylarger quantity of steam in order to effect vaporization; and eventuallya maximum point of stripping effect is reached beyond which furtherelimination of the light materials is not eflected regardless of theamount of stripping steam used.

We have discovered that, in side stream steam stripping, a moreeffective stripping operation can be accomplished if a more intimatemixture of the steam with the side stream is formed and if, at the timethe mixture of steam and oil is effected, sufiicient heat is supplied tothe mixture to prevent a temperature drop therein because of thevaporization induced by the steam. The method and apparatus forutilizing this.discovery will be described hereinafter.

It is an object of our invention to provide an improved method of sidestream steam stripping characterized by simplicity, efficiency, andeconomy of operation.

An important object of the invention is to provide a method of sidestream stripping which contemplates intimately mixingsteam with thepartially stripped side stream obtained from the lowest contactingelement of a stripping column, passing the resulting mixture in acomparatively restricted stream through a heating zone in whichsuilicient heat is supplied to the mixture to prevent a drop in thetemperature thereof because of vaporizationof the lighter componentsthereof, thenseparating the steam and the remaining low boilingcomponents from the partially stripped side stream by releasing themixture from the restricted heating zone, and countercurrentlycontacting the steam so released with the incoming side stream in itspassage through the stripping column.

Another object of the invention is to provide an improved method ofsupplying heat to a side stream undergoing stripping from a hotcirculatingstream, preferably taken from the main fractionating column,which circulating stream may be additionally heated prior to its use inthe stripping unit to a sufiiciently high temperature level to provide asuitable temperature difference for a flow of heat to the side streamundergoing stripping. Since the circulating stream may be taken in anydesired quantity, it ofifers a convenient and flexible heat source.

A further object of the invention is the provision of a method whichcontemplates successively correcting the boiling range of a plurality ofsidestream fractions by the application of indirect heat to a pluralityof stripping units arranged in parallel, series, or series-parallelrelation with respect to the application of the heat source.

A special object is the provision of a method for side stream steamstripping whereby considerable flexibility in the application of anindirect heat source to the side stream during stripping is permitted.

Further objects and advantages of our invention will be apparent fromthe following description of a preferred form of embodiment thereoftaken in connection with the attached drawing, which is a diagrammaticflow sheet of a process in which the invention may be carried out.

The principal feature of our invention relates to what may be roughlyregarded as two phases of side stream stripping. The first phase may besaid to consist of contacting countercurrently a side stream with steamin a stripping unit provided with a heating zone of restricted area. Thesecond phase may be said to consist of intimately mixing steam with thepartially stripped side stream after removal thereof from thecountercurrent contacting Zone, maintaining the resulting mixture duringpassage thereof through the heating Zone at a sufficiently hightemperature level to avoid a drop in temperature thereof caused by thevaporization effect produced by the mixing of the steam with the oil,and then utilizing such steam in countercurrently contacting the sidestream as contemplated by the first phase.

The apparatus for carrying out this feature of the invention maycomprise a stripping column and a reboiler, which may be located eitherinternally or externally of said column and is interconnected to thelower portion thereof. As the process is carried out, a side stream iswithdrawn from the main fractionating column and is introduced into thetop of the stripping column, the upper portion of which is provided withbubble plates or similar contact devices. The partially stripped sidestream is removed from the lower-most bubble plate and is mixed withsteam; the resulting mixture is passed through the reboiler, from whichit is then released, and the steam then passes upwardly incountercurrent relation to the descending side stream. Because of therestricted area in which the mixture of steam and oil is effected, amore intimate mixture of these substances is obtained whereby moreefiective vaporization of the light ends is promoted. Bysupplying heatto the mixture as it passes through the reboiler, the temperaturethereof is prevented from dropping with the result that greatly improvedstripping is obtained.

The source for supplying heat to the reboiler may consist of acirculating stream taken from the main fractionating column. If thisstream does not have a high enough temperature, it may be additionallyheated before being introduced into the reboiler so that it has asufficiently high temperature level to allow the transfer of heat to theside stream to compensate for the cooling resulting from thevaporization produced by the partial pressure effect of the steam.

A plurality of side stream fractions may be withdrawn from the mainfractionating column; in such case, the respective stripping units maybe arranged so that the circulating stream supplies heat to all of them.As already stated, the circulating stream may be passed in parallel,series, or series-parallel relation to the stripping units. In this wayit is possible to use the same circulating stream for operating theseveral stripping units.

The fact that the circulating stream may be taken from the mainfractionating column in suficient quantity as the necessary heatingmedium for the reboilers greatly simplifies our process. Furthermore, byeffecting a further cooling of the circulating stream to a temperaturelower than that at which it was removed from the main fractionatingcolumn, we obtain a. convenient and expeditious method for providing anintermediate refluxing of the main fractionating column. As is wellknown in the art, this is a desirable expedient in order to reduce thesize of the fractionating column.

Referring to the accompanying drawing we have illustrated our inventionas applied to the fractional distillation of a hydrocarbon oil or amixture of hydrocarbons although it is equally applicable to thefractional distillation of mixtures of other types of liquids. As is thecustomary practice, the hydrocarbon oil, suitably heated in a heater I,is charged to fractionating column 2 through line 3. The oil ispreferably heated to such a temperature that it may be flashed to thedesired extent in the lower portion of the column. The flashed Vaporsare then separated into fractions, which are removed as an overheadstream as at 4 and as side streams as at 5, 6, and i. To accomplish thisfractionation, the tower 2 may be equipped with suitable bubble decks 8or the like. Reflux, comprising a portion of the overhead condensatefrom condenser 9, is introduced into the tower 2 at I0. The remainder ofthe overhead condensate is removed at H as the overhead product. Theliquid portion of the heated oil flows downwardly over bubble decks [2,by means of which any remaining low boiling constituents are separated.To aid in this separating action, steam may be introduced at l3. Thestripped residual oil is rcmoved as a bottoms stream at M.

The side streams removed at 5, 6, and 1 represent certain fractions ofthe hydrocarbon oil which are desired as end products. Even though afairly close fractionation may be accomplished in the tower 2, theseside streams invariably con tain an appreciable amount of low boilingmaterial which must be removed before the fractions can meet productspecifications and market requirements.

In accordance with our invention, a side stream withdrawn as at l ispassed through line 55 to a stripping apparatus which may comprise astripping column !6 having connected to the lower portion thereof areboiler 17. As is characteristic of a conventional stripping column,bubble plates it are located in the upper portion of the strippingcolumn it. The side stream removed at 1 flows downwardly over the platesl8 whereby it is subjected to the stripping effect of steam introducedas will hereinafter be described for the removal of light ends. Thepartially stripped side stream is removed from the lowest plate throughline l9 and is then introduced into the base of the reboiler Ill. Thereboiler I! may be of a conventional type provided with a bundle oftubes forming a heating zone enclosed within an outer shell, throughwhich a heating medium circulates. The heating medium circulated throughthe shell side of reboiler I! may be supplied from any suitable sourcewhich has a sufficiently high temperature so that a suitable temperaturedifference can be obtained and the quantity of which is suflicientlygreat so that an adequate amount of heat is provided. As already pointedout; under proper conditions a stream of oil may be continuouslycirculated from column 2 through the reboiler.

Stripping steam from line 20 is mixed with the partially stripped sidestream in line l9 before it enters reboiler II. The introduction of thesteam into the side stream in this manner provides for the formation ofa more intimate mixture. By passing this mixture through a confined orrestricted zone such as afforded by the tube side of the reboiler, wecan obtain the more intimate mixture and can simultaneously providesufficient heat to maintain the cooling effect resulting fromvaporization of the remaining light ends inbalanced relation with thepartial pressure effect produced by the steam. In this way a substantialimprovement is obtained in the stripping operation.

The mixture of the steam and the side stream is returned by line 2| tothe stripper l6 at a point in the lower portion thereof. The release ofthe mixture from the restricted zone effects a separation of the steamand the remaining light ends from the side stream. The steam and vaporspass upwardly through stripper l6 wherein the steam countercurrentlycontacts the side stream introduced through line l5 for the removal of aportion of the light ends therefrom. The finished side stream iswithdrawn from the base of the stripper l6 through line 22, and thestripped vapors and steam are returned to tower 2 by means of line 23.

It is frequently the practice to withdraw a plurality of side streamsfrom a fractionating col umn for boiling range correction treatment. Insuch case, as indicated in the drawing, stripping units identical withthat already described may be used. Where a plurality of side streams isthus treated, however, it is convenient to provide a single heatingsource for all the stripping units. 7 In many instances a stream of oilat a sufficiently high temperature and in sufiicient quantity is notavailable at a particular plant for this heat exchange duty. If such isthe case, a stream may be withdrawn as at 24 for fractionating column 2for circulation in a predetermined manner through the reboilers of theseveral stripping units. It is preferable to withdraw such stream at apoint above the point of withdrawal of the lowest side stream asindicated. The use of such a stream provides a flexible heat sourcesince the quantity of the stream can be readily varied. Inasmuch as thestream withdrawn at 24 is lower in temperature than the lowest withdrawnside stream, it is passed through line 25 by means of pump 21 to heater28 wherein its temperature is sufliciently raised so as to provide asuitable temperature difference for the transfer of heat to the sidestream in reboiler l'l. It will be appreciated that heater 28 and heaterI may be positioned in the same furnace setting. It will also beappreciated that the quantity, if not the temperature, of other streamssuch as the bottoms stream l4 will not ordinarily be suflicient toprovide an adequate source of heat for this purpose.

The reboilers for the several stripping columns l6, lfia, and IE2) arepreferably connected in parallel with respect to the circulating streamor heating medium in line 24 because of the greater degree of controland flexibility afforded by this system. In such case, a portion of thecirculating stream is passed through line 29 from manifold 24a into theshell side of reboiler ll wherein a transfer of heat to the mixture ofsteam and oil passing through the tubes thereof is effected. Thecirculating heating medium is discharged from reboiler through line 30into the return manifold 3|.

Another portion of the circulating stream is passed through line 33 frommanifold 240. into reboiler l'la. In this case heat is transferred tothe mixture of the partially stripped side stream withdrawn through lineIM and the steam introduced through line 20a. while the mixture ispassing through the restricted tube-side heating zone of reboiler Ho.The mixture, properly heated, is then discharged into column lfia bymeans of line Zla. The finished side stream is removed at 220., and thesteam and stripped vapors are returned to column 2 through line 23a. Theheating medium is returned through line 36 to'the return manifold 3|.

The remainder of the circulating stream is passed through line 31 frommanifold 2 1a into reboiler l'lb, in which-heat is transferred to themixture of the partially stripped side stream passed through line lilband the steam introduced through line 20b. The heated mixture is thenpassed through line 2 lb into column l6b, from which the stripped sidestream is removed at 22b. The steam and stripped vapors are returned tocolumn 2 through line 23b. Line 39 is provided for the discharge of theheating medium from the shell side of reboiler l'ib into the returnmanifold 3|.

For eflicient stripping operation, control valves 4|, Ma, and 4|b may beprovided for the regulation ofthe amount of heating medium passedthrough each reboiler. In this way the heat transfer from thecirculating stream to each side stream may be nicely controlled, and theproper balance between the cooling effect due to the vaporization andthe partial pressure effect produced by the steam may be maintained. Avalved by-pass 42 may be provided so that any excess heating medium neednot be circulated through any of the reboilers.

The circulating heating medium is returned to column 2 through line 43.In the usual case the temperature of this heating stream after passagethrough all the reboilers will be about the same or somewhat lower thanthe temperature at which it was withdrawn from column 2. Accordingly,the circulating stream may then be returned to column 2 as at 45a. Inthis manner a stream of oil may be continuously circulated as theheating medium for the several stripping units.

This method of providing a heat source for the stripping operations alsohas the advantage that an intermediate refluxing or cooling of the mainfractionating column may be simultaneously provided. When intermediaterefluxing is to be used, the circulating heating medium, after passagethrough the several reboilers, is passed through exchanger or cooler 48in line 43. In such case the circulating stream is cooled to atemperature considerably below that at which it is removed from column2. The additionally cooled circulating stream is then returned to column2 preferably at a point as at 45b above the point of withdrawal. Theeffect of this intermediate refiuxing or cooling is to reduce thecondensing load at the top of column 2 and to reduce the size of column2 as is well known in the art. It will be noted that line 49 is providedto by-pass the circulating stream around cooler 48 when it is notdesired to operate column 2 with the intermediate reflux.

A stream of .oil or other fluid, external to the distillation system,may also be used as the circulating heating medium provided such streamis available not only in sufficient quantity but also at a temperaturesufiiciently high to provide an adequate temperature difference toeffect a heat transfer in the several reboilers. In such case, thisstream is introduced into the heat exchange system through line 5| and.will be circulated through the several reboilers for heat exchangepurposes in accordance with the predetermined arrangement. After passagethrough the several neboilers, the stream may be conveniently withdrawnfrom the system as at 52. As already pointed out, however, such a streamwill not usually be available in the ordinary plant.

Although the heating medium is preferably circulated through the severalreboilers in parallel as described because of the greater flexibilityprovided, it may also be circulated through the reboilers in seriesrelation if such operation is found desirable. In such case, lines 54and 55 are provided so that a series fiow can be accomplished. Thecirculating stream then flows from line through line 28 to reboiler ll,through line 30 to return manifold 3| and through lines 54 and 33 toreboiler Ila, through line 36 to return manifold 3| and through lines 55and 31 to reboiler Nb, and through line 39 and return manifold 3| toreturn line 43. Preferably valved line 57 and valves 58 and 59 areprovided so that the amount of heating medium passed'through eachreboiler may be controlled and so that flexibility of operation may beobtained.

Circulation of the heating medium through the several reboilers inseries-parallel relation may also be practiced if desired. Appropriatelypositioned valves as indicated in the drawing are provided so that anyparticular flow of the circulating stream may be accomplished.

As previously indicated, reboiler l! for stripping column l6 may bepositioned either externally or internally thereof. The ordinaryarrangement of the apparatus is as shown in the drawing. Our invention,however, may also be carried out in a stripping column provided with aninternal reboiler adapted to heat the partially stripped side stream ina zone of restricted area.

Our invention is particularly adaptable to the fractional distillationof hydrocarbon oils and the like. It may be employed in conjunction withpressure, atmospheric, or vacuum distillation. It may be used in thedistillation of crude' oils, reduced crudes, and other mixtures ofhydrocarbon oils. In all these cases our invention provides an improvedmethod of obtaining a superior side stream product.

Our invention finds particular application in the vacuum distillation ofa reduced or topped crude such as a Mid-Continent reduced crude for theproduction of lubricating oil fractions and asp-halt. In this case it iscustomary to withdraw several side streams from the vacuum distillaticncolumn, each of which streams contains a small percentage of undesirablelowboiling constituents. Frequently these low boiling componentscomprise from 3 to 5% of the side stream. Such small percentages aresuificient, however, to lower the flash point of the lubricating oilfractions materially below product specifications. By means of ourinvention, these low boiling constituents may be effectively removed,and lubricating oil fractions having a quality superior to that offractions obtained by customary side stream stripping can be produced.

The following operating data on such a vacuum distillation of a typicalMid-Continent reduced crude are given by way of example. The quantitiesare based on a charge of bbls. of reduced crude, which is introducedinto column 2 at a temperature of 760 F. The stream removed at Mcomprises 15 bbls. of asphalt at 700 F. As in the process described,three finished lubricating oil side streams are produced. The lowermostside stream comprises 31 bbls. of a cylinder stock having a viscosity of150 Saybolt Universal seconds at 210 F.; it is removed at a temperatureof 625 F. from stripper I6. The intermediate side stream comprises 12bbls. of a viscous neutral oil whose viscosity is 55 S. U. seconds at210 F.; it is removed at a temperature of 550 F. from stripper Mia. Theuppermost side stream comprises 15 bbls. of a non-viscous neutral oilhaving a Viscosity of S. U. seconds at 100 F.; it is removed fromstripper l6?) at a temperature of 440 F. The overhead comprises 27 bbls.of a heavy gas oil. The circulating stream removed at 24 has atemperature of 600 F. and is heated to 740 F. before being passedthrough the several reboilers; it is returned to column 2 at 4511 at atemperature of 600 F. On the same basis, this circulating streamconsists of approximately 35 bbls.

Furthermore, in such vacuum distillation there are no available externalfluid streams which could be used as the circulating heating medium.Accordingly, in such a situation the proposed method of continuouslycirculating a stream of oil not only provides a convenient, readilyavailable heating medium but also permits advantage to be taken of acirculating intermediate reflux stream. In addition, the bottoms asphaltstream is not sufficient either in quantity or in temperature to be usedas the heat source; it is also undesirable for such purpose because ofits poor characteristics for eilicient heat transfer.

It will be seen that our improved process combnes the advantages of sidestream steam stripping and side stream reboiling and at the same timeeliminates the disadvantages of each. The temperature of the side streamis not lowered as in ordinary steam stripping since the heat input fromthe reboiler supplies the heat of vaporization of the undesiredconstituents and prevents a drop in temperature from occurring. In likemanner, since the partial pressure effect of the steam permits the lowboiling constituents to vaporize at a lower temperature, only enoughheat must be supplied to take care of the heat of vaporization of theundesired constituents at the lower effective pressure; it is notnecessary as in customary reboiling practice to raise the temperature ofthe side stream in order to efiect flashing and vaporization of theundesirable low boiling constituents.

Results similar to those obtained by the improved process describedabove may also be produced in a stripping column provided with aninternal reboiling coil if the steam is introduced into the oil wellbelow the reboiling coil so that the steam and vapors, in rising throughthe oil in a plurality of column-like relatively restricted paths, forman intimate mixture with the oil that is maintained at a relativelyconstant temperature by the heat supplied from the reboiling coil.

It will be apparent that modifications such as the substitution of aninert gas as the stripping medium in place of steam may be made. It isdesired, therefore, to cover all such modifications distillation zonefrom which. a fraction of the oil is withdrawn as a side stream and issubjected to an independent boiling range correction treatment for theremoval of retained undesirable low boiling components, which arepartially removed by countercurrent contact of the side stream withsteam in a stripping unit provided with a heating zone, the improvementwhich comprises withdrawing the side stream, partially stripped, fromthe lower portion of the stripping unit, mixing steam with saidpartially stripped side stream to eiiect vaporization of the remaininglow boiling components therefrom, passing the resulting mixture in a,comparatively restricted stream through said heatingzone, simultaneouslysupplying heat to the mixture in an amount sufficient to prevent a dropin the temperature thereof because of the vaporization of the lowboiling components, which temperature drop would deleteriously affectthe stripping operation, and returning said mixture to the strippingunit at a pointbelow the point of withdrawal of the partially strippedside stream.

2. In the distillation of a hydrocarbon oil'in adistillation zone fromwhich a fraction of the oil is withdrawn as a side stream and issubjected to an independent boiling range correction treatment for theremoval of retained undesirable lowboiling components, whichare-partially removed by countercurrent contact of the side stream withsteam in a stripping unit provided with a reboiler, the improvementwhich comprises withdrawing the side stream, partially stripped, fromthe lower portion of the stripping unit, forming an intimate mixture ofsaid partially stripped side stream with steam, said steam serving toeffect vaporization of the remaining low boiling components therefrom,passing the intimate mixture in a comparatively restricted streamthrough said reboiler and simultaneously supplying heat to the mixturein an amount sufficient to prevent a drop in the temperature thereofbecause of the vaporization of the low boiling components, whichtemperature drop would deleteriously affect the stripping operation, andreturning said mixture to the stripping unit at a point below the pointof withdrawal of the partially stripped side stream.

3. In the distillation of a hydrocarbon oil in a distillation zone fromwhich a fraction of the oil is continuously withdrawn as a side streamand is subjected to an independent boiling range correction treatmentfor the removal of retained undesirable low boiling components, theimprovement which comprises continuously countercurrently contacting thewithdrawn side stream with steam in a stripping unit in order tovaporize a portion of said low boiling components therefrom, saidstripping unit being provided with a heating zone, withdrawing thepartially stripped side stream from the lower portion of the strippingunit, mixing steam with said partially stripped side stream to eiiectvaporization of. the remaining low boiling components therefrom, passingthe resulting mixture in a comparatively restricted stream through saidheating zone, simultaneously supplying heat to the mixture to maintainthe temperature thereof sufficient for the desired stripping, returningsaid mixture to the stripping unit at a point below the point ofwithdrawal of the partially stripped side stream,

component vapors from said mixture, permitting saidsteam to passupwardly through the stripping'unit for continuous countercurrentcontact with the withdrawn side stream, removing the steam and the lowboiling component vapors from the top of the stripping unit, andremoving the stripped side stream from the bottom of the stripping unit.

4.. In the distillation of a hydrocarbon oil which includes the stepsof'fractionating the oil in a iractionating zone, withdrawing a fractionof the oil as a side stream from the fractionating zone, and subjectingthe withdrawn side stream to an independent boiling range: correctiontreatment forthe removal of retained undesirable low boiling componentsin a stripping unit provided with a reboiler, the improvement whichcomprises withdrawing the side stream, partially stripped, from thelower portion of the stripping unit, forming an intimate mixture of saidpartially stripped side stream with steam, said'steam serving to efiectvaporization of the remaining low boiling components therefrom, passingthe intimate mixture. in a comparatively restricted stream, through saidreboiler, separately withdrawing from the distillation zone a secondstream for use as aheating medium, passing said second stream inindirect heat-exchange relation in saidv reboiler with saidsidestream-steam mixture to supply heat thereto in order to prevent a dropin the temperature thereof because of Va porization of the low boilingcomponents, returning said second stream. to the distillation zone at apoint above its point ofwithdrawal, and returning said side stream-steammixture to the stripping unit at a point below the point of withdrawalof the partially stripped side stream.

5. In the distillation of a hydrocarbon oil in a distillation zone fromwhich a plurality of separate fractions of the oil is removed as sidestreams and subjected to independent boiling range correction treatmentfor the removal of retained undesirable low boiling components inindividual stripping units provided with heating zones, the improvementwhich comprises withdrawing each side stream/partially stripped, fromthe lower portion of its respective stripping unit, mixing steam witheach partially stripped side stream to effect the vaporization of theremaining low boiling components therefrom, passing each resultingmixture in a comparatively restricted stream through its respectiveheating zone, separately withdrawing from the distillation zone anotherstream for use as a heating medium, additionally heating said otherstream to a temperature higher than that of the lowest Withdrawn sidestream, passing said additionally heated stream in indirect heatexchange relation through said heating zones with said side streamsteammixtures to supply heat thereto in order to prevent a drop in thetemperature thereof because of vaporization of the low boilingcomponents, returning said other stream to the distillation zone at apoint above its point of withdrawal, and returning each sidestream-steam mixture to its respective stripping unit at a point belowthe point of withdrawal of the respective partially stripped sidestream.

6. In the distillation of a hydrocarbon oil in a distillation zone fromwhich a plurality of separate fraction of the oil is removed as sidestreams and subjected to independent boiling range correction treatmentfor the removal of retained undesirable low boiling components inindividual stripping units provided with reboilers, the improvementwhich comprises withdrawing each side stream, partially stripped, fromthe lower portion of its respective stripping unit, forming an intimatemixture of each side stream with steam, said steam serving to effectvaporization of the remaining low boiling components therefrom, passingeach resulting mixture in a comparatively restricted stream through itsrespective reboiler, separately withdrawing from the distillation zoneanother stream for use as a heating medium, additionally heating saidother stream to a temperature higher than that of the lowest withdrawnside stream, passing said additionally heated stream in indirect heatexchange relation through said reboilers with said side stream-steammixtures to supply heat thereto in order to prevent a drop in thetemperature thereof because of vaporization of the low boilingcomponents, cooling said other stream to a temperature below that atwhich it was withdrawn from the distillation zone, returning said cooledother stream to the distillation zone at a point somewhat above itspoint of withdrawal, thereby providing an intermediate reflux for saiddistillation zone, and returning each side stream-steam mixture to itsrespective stripping unit at a point below the point of withdrawal ofthe respective partially stripped side stream.

7. In the vacuum distillation of a reduced crude in a distillation zonefor the production of a plurality of lubricating oil fractions, whichare removed from the distillation zone as side streams and subjected toindependent boiling range correction treatment for the removal ofretained undesirable low boiling components in individual strippingunits provided with reboilers, the improvement which compriseswithdrawing each lubricating oil fraction, partially stripped, from theindirect heat exchange relation with said lubrieating oil fraction-steammixtures to supply heat thereto in order to prevent a drop in thetemperature thereof because of vaporization of the low boilingcomponents, returning said other stream to the distillation zone at apoint above its point of withdrawal, and returning each lubricating oilfraction-steam mixture to its respective stripping unit at a point belowthe point of withdrawal of the respective lubricating oil fraction.

8. The improved distillation method as claimed in claim 5, whichincludes passing the heating medium stream through the several heatingzones in parallel relation thereto.

9. The improved distillation method as claimed in claim 5, whichincludes passing the heating medium stream through the several heatingzones in series relation thereto.

WHEATON W. KRAFT. WALTER L. BASS.

